Originally, most tooling platforms on robots had only one tool (the word tool here being used in a very general sense to indicate any device operable to carry out some work function). However, it is now increasingly common to have several tools on the same tooling platform, each tool operated and controlled by its own pneumatic, hydraulic or electric controls, as well as appropriate feedback systems. Traditionally, each tool has had at least two sensors to indicate its position. Thus if there were four tooling mounts on a tooling platform, eight position sensors were used. Some systems have also used two feedback sensors for each tool. Others have used an individual pneumatic position controller for each tool.
The result has been an evolution of systems having excessive mass and excessive complexity, with many valves, and a number of pneumatic servo-controllers.